Frequency hopping is a radio communication technique in which information is transmitted using a sequence of carrier frequencies that change at various times (i.e., hop) in center frequency over the available spectrum. Of primary interest for the subject invention is the technique of slow frequency hopping in which the hop rate is much less than the information symbol rate and thus many symbols are sent on the same carrier frequency during each hop, maintaining narrowband transmission conditions within each hop.
In a single or multicellular wireless communication network, each cell has a single leader station that coordinates communication among the remote stations that are within its cell. Specifically, the leader station provides, either implicitly or explicitly, the control and timing information that is required for the remote stations to remain in frequency-hopping synchronization with the leader station. In a single cell of such a wireless communication network, multiple remote stations need to access the shared radio channel. In many wireless Local Area Networks (LAN), a Carrier Sense Multiple Access (CSMA) type protocol is used, in part, because of its low access delays under low and moderate traffic loads. None of the prior art on synchronization in frequency-hopping systems that use CSMA type protocols deals with the problem of achieving fast frequency acquisition and of maintaining Frequency-hopping synchronization in an environment that is subject to radio transmission errors (e.g., due to interference or fading).
The following references are typical of the background art in the field of synchronization techniques in Frequency-hopping systems.
U.S. Pat. No. 5,287,384 issued Feb. 15, 1994 to Avery et al. entitled "Frequency Hopping Spread Spectrum Data Communications System" discloses a frequency-hopping communication system in which a time-slotted Medium Access Control (MAC) protocol is used and provisions for low-power operation modes are made. To achieve initial frequency synchronization, the remote stations listen for a time mark frame that is generated periodically by the base station.
U.S. Pat. No. 5,130,987 issued Jul. 14, 1992 to Flammer entitled "Method for Synchronizing a Wide Area Network without Global Synchronizing" discloses a frequency-hopping packet communication system that does not use a master clock or master control unit, but instead makes use of a receiver's frequency-hopping timing and identification to control communication. In this scheme, each receiving station establishes a table of receiver frequency-hopping sequence offsets of each other station within its communication range, and each station announces its presence on each frequency in a packet with a hop timing offset indicator.
U.S. Pat. No. 4,872,182 issued Oct. 3, 1989 to McRae et al. entitled "Frequency Management System for Use in Multistation H. F. Communication System" discloses a system for managing frequencies in a frequency-hopping communication system. Any station desiring to communicate with another station of the network, repeatedly transmits, on each of the K communication frequencies in sequence, a probe message comprised of three successive symbols. Each station that is in an idle mode monitors the level of activity of each of the K communication frequencies by successively dwelling on each frequency for the length of time required for any message-transmitting station to step through all K frequencies in the network.
U.S. Pat. No. 4,850,036 issued Jul. 18, 1989 to Smith entitled "Radio Communication System Using Synchronous Frequency Hopping Transmissions" discloses a frequency-hopping synchronization scheme in which a special start-up sequence with a special control frequency is used by the master station to communicate the frequency-hopping sequence that will be used.
U.S. Pat. No. 4,677,617 issued Jun. 30, 1987 to O'Connor et al. entitled "Rapid Frequency-Hopping Time Synchronization" discloses a method for synchronizing the operation of frequency-hopping communication devices. In this method, a special control station (i.e., the network device) transmits sequences of synchronization messages at a transmit hop rate that is substantially higher than the receive hop rate so that all possible receive frequencies and synchronization codes are bracketed by the transmission sequence. During the acquisition phase (called the idle state in the patent), a remote station hops through a discrete set of receive frequencies for a dwell time that is determined by the hop rate. In this way, the two stations can achieve rapid synchronization.
U.S. Pat. No. 4,558,453 issued Dec. 10, 1985 to Mimken entitled "Synchronization Method and Frequency Hopping Communication System" discloses a method for synchronizing two frequency-hopping radios. In this method, a transmitter automatically initiates the transmission of a predetermined number of cycles of a synchronization signal at each of a number of different frequencies. The receiver is being tuned through the same frequencies at a relatively slower rate so that the synchronization signal is received at each of the receiver frequencies.
A patent application filed on Oct. 22, 1993, serial number 8-142555, by Bauchot et al. entitled "Radio Communications with Fault Tolerant Frequency Hopping Synchronization" discloses a method for acquiring and maintaining frequency-hopping synchronism in the presence of radio transmission errors. The latter patent is hereby incorporated herein by reference. To achieve initial frequency synchronization, a remote station listens for control information that is broadcast by the base station.
Therefore, there is a need for a frequency-hopping communication system that uses a CSMA protocol and provides methods of achieving fast frequency acquisition and of maintaining frequency-hopping synchronization in an environment that is subject to radio transmission errors.